Most dot matrix printers utilize a print head mounted on a carriage that is moved across the paper or other record sheet to print a line of characters. In some instances, the print head provides a complete array of print rods or other print elements, one for each position in the dot matrix; in others there is just one column of print elements and each character is reproduced by a series of column-increment steps of the print head. These printers are somewhat limited in speed of operation because the print head must be stepped completely across the record sheet to reproduce each line of characters. In addition, printers of this type have inherent acceleration and deceleration problems, which increase markedly for high print rates and which lead to difficulties in maintaining adequate quality in the reproduced characters.
Dot matrix line printers are also known in the art. A dot matrix line printer may provide a full complement of print elements at each character position along the line. Alternatively, there may be a single column of print elements for each character position, the print elements being shifted horizontally through a series of column-increment steps, corresponding to the number of columns in the dot matrix, in printing each line. For these printers, however, particularly when print rods or other impact print elements are employed, costs may be inordinately high due to the large number of print elements and print element actuators involved. Thus, for a conventional line of eighty characters, using a complete set of print elements for a simple 5.times.7 matrix at each character position, there are twenty-eight hundred print elements, each requiring its own actuator. For a line printer having only a single column of print elements at each character position, the eighty-character line requires five hundred sixty print elements and five hundred sixty actuators, still an excessive number.
Another form of dot matrix line printer uses just one print element per character position. The economy of construction is obvious; only eighty print elements and eighty actuators are required to print a complete line of text. A dot matrix line printer of this general kind is described in Howard et al U.S. Pat. No. 3,802,544, issued Apr. 4, 1974, which constitutes the most pertinent prior art known to the inventor relative to the present invention. This type of printer, however, presents substantial difficulties with respect to the control system that supplies the requisite dot print signals to the print element actuators and that controls relative movements between the print elements and the record sheet.
Thus, in the Howard et al printer the print elements are continuously cyclically moved parallel to the print line, first in a forward (left-to-right) direction and then in a reverse direction, that cyclical movement spanning all of the column positions of the matrix. The data words representative of one full line of print are initially translated, by a character signal generator, for one dot position in the matrix, to develop eighty dot position signals that are recorded in a buffer register. When the print elements are aligned with the first position (column one, row one) in the matrix, all of the dots for all characters are printed for that position. Before the print elements reach the next column position in the first row, the data words are again translated to provided a new set of dot position signals in the buffer register so that all of the dots or the second matrix position can again be printed simultaneously. This process is repeated for each column of the matrix to finish the first row, after which the print elements are cycled back in the reverse direction without printing and the procedure is again repeated for each column position in the second row of the matrix. The record sheet is inclined slightly to the line of the print elements and is advanced continuously to afford the requisite spacing between matrix rows. Thus, the control system must coordinate continuous movements of the print elements and the record sheet and the application of signals to the print element actuators through a total of thirty-five individual steps in printing one line of characters, when using a 5.times.7 matrix. For a larger matrix (e.g. 7.times.9) the number of steps is, of course, much larger.
Coordination and timing in the system of the Howard et al patent is achieved by a series of countdown circuits supplied from a clock source that actuates a register having a storage capacity of one line of characters. This presents the possibility that, if any one of the several countdown circuits misses a single count, an entire line of characters can be distorted. At the same time, if the separate drives for the record sheet and the print elements are the least bit out of synchronism with the print element actuator controls, substantial distortion of the characters may be experienced. Thus, there is a distinct need for a positive control system for a dot matrix line printer of this general kind, a control system that affords positive control of each of the thirty-five or more stages in the printing of the line of characters such that is any single operation occurs asynchronously, subsequent operations will automatically return to synchronization. Further, there is a need for positive control of the physical movements of the print elements and the record sheet to assure accurate location of the dot positions in the matrices that constitute the individual characters.